Some important aspects of thorium metabolism and their effects on urinary excretion levels

The current bio-assay procedures employed to monitor the Th exposure in occupational subjects, are based on the ICRP metabolic model of Th, which in turn is based on the animal experiment and hence may not be valid for humans. With a view to study its metabolic behaviour in humans, the concentrations of Th is human tissues and body fluids were determined using radiochemical neutron activation analysis (RNAA). Some of the important observations of this study are: 1) The Th concentration in human skeletal tissue increased with the age of the subjects, 2) The observed daily urinary excretion of Th for the occupational subjects was much lower than that expected on the basis of ICRP metabolic model and the Th body burden of those subjects, and 3) The observed ratio of liver to skeletal burden of Th was found to be much higher than that expected on the basis of ICRP model. This study indicates that there is a need to revise the ICRP metabolic model of Th and that caution should be exercised when using the derived excretion limits recommended in ICRP-54 to interpret the Th exposure to the occupational subjects on the basis of the daily urinary excretion of Th. (author). 4 refs., 2 figs., 3 tabs